You should try with this code, Hope it is worked

<report
string="Purchase Order"
id="purchase.action_report_purchase_order"
model="purchase.order"
report_type="qweb-pdf"
name="purchase.report_purchaseorder"
file="purchase.report_purchaseorder"
print_report_name="
(object.state in ('draft', 'sent') and 'Request for Quotation - %s' % (object.name) or
'Purchase Order - %s' % (object.name))"
menu="False"
/>

Restart the server and update the module.